Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.
Q: I have converted a half-dozen 1974 to 1976 Honda CB550s to café racers. The first two went to my two sons. My current problem revolves around the petcock on a 1974 tank. This particular unit is specific to the 1974 model. Later years used a 20mm bung and a completely different style petcock. The original had a rusted out bowl, so I purchased a repro from a reputable supplier. I always test new components before installing them. On the bench I have the tank full of fuel, petcock installed and a catch basin underneath. When the unit is put in reserve or on, the fuel flows out of one port only. I am sure you are familiar with the 4-carb setup on the little CB’s, each port supplies two carbs in the set. If I put my finger over the flowing port, the other starts to dispense fuel. I can move my finger back and it starts to flow out of the open port. The engine will not run properly with this problem. Two carbs in the set are always short on fuel.
I had this problem on another 1974 tank on a build several years back. It was solved by cutting out the petcock mount and attaching a bung from a 1975 tank. Then I installed a 1975 petcock. I would like to keep this bike as stock as possible, so I am taking some time to try to solve this problem. The supplier is sending me an OEM part to replace the repro, and I am hoping for a good result — two ports flowing simultaneously. Based on the problems encountered on the last 1974 that I worked on, I fear that the OEM unit will not help. Is it possible that the system is designed to fill two carbs and then switch to filling the other two? I can’t see how this would work, particularly at any throttle opening above half? Any thoughts you have would be greatly appreciated. I always look forward to your column.
Gerald Delaney/via email
A: I think that once the carburetor bowls are full and all the air bubbles work their way back up to the petcock and tank, the lines from the petcock will act more like a siphon and hydraulic pull will cause them to both flow evenly. You could test this by hooking the petcock up to the carburetors and remove the bowls from one carburetor on each feed line. Holding the floats up on the open carburetors should allow the other two to fill and stop the flow. You could then release the open bowl floats and see if they drain an equal amount over time into separate containers.